The present invention relates to digital image display or generation of images, and more particularly concerns methods and apparatus providing increased data compression ratios with large numbers of different irregular patterns.
In the manufacture of printed circuit boards, master images are employed in the processing of the conductive patterns on the board. The master image comprises a number of pads, relatively large conductive areas, and a number of interconnecting conductive leads or traces. Increased density requires leads of smaller width, closely packed with each other and with the pad patterns. The traces are often of a relatively uniform rectangular shape, running vertically, horizontally or diagonally in straight or simple curved lines, whereas the pad patterns may be of irregular shape such as circular, annular, triangular or other nonorthogonal configurations. The required high density and high resolution require that the image generator be capable of high-speed precision definition of traces and patterns.
Graphic plotters and tape-up methods have been employed to generate master images but both require large amounts of time. Computer controlled lasers for generating images significantly reduce the image printing time, but require excessively large storage as display resolution increases. The laser writing beam is caused to sweep the writing medium in a substantially rectangular raster. The beam, of about 1 mil in diameter, is turned on and off over image elements (pixels) of about 1 mil in length. Accordingly, to generate the scan line data for an 18".times.24" digitally produced image there is required about 432 megabits of data storage. The amount of data that must be stored for a particular image may become the limiting factor in reducing imaging time because of practical and economic constraints on the rate of data transfer, which data rate is inversely proportional to the time required to form an image.
In an article entitled "The Primary Pattern Generator" in the Bell System Technical Journal of November, 1970, pages 2033-2074, there is described a laser pattern generator in which the bit image of the scan line to be generated is completely assembled in a buffer before the start of a given scan line. Recognizing the great amount of data that must be handled, since each line in this system consists of twenty six thousand bits which must be taken from the buffer in serial fashion, part of the information is compressed according to well known coding techniques, sometimes termed two-dimensional or 2D coding. In this type of data compression, data commands define only changes to be made to currant scan lines. If there is no change in a succeeding scan line, the latter is merely repeated and no data commands for such unchanged line are transmitted. Such an approach is effective on orthogonal patterns that change in either horizontal or vertical directions. Nevertheless, on irregularly shaped patterns and patterns of other geometric shapes, such as a circle, ring, triangle or the like, which require a change in almost every scan line, the advantages of this type of compression are lost. Possibly because of this difficulty, the system of the Bell Technical Journal article produces data that contains all of the twenty six thousand bits for a new scan line, rather than merely update commands, for those instances in which a great number of update commands would be required to produce the succeeding scan line. Thus, exceedingly large amounts of data still must be handled.
Another coding technique is run length coding in which length of the scan between changes in the digital level is stored, instead of storing data defining each individual digital level. Accordingly, in the use of run length coding seven bits may define a continuous run of as many as one hundred twenty-seven zeros or ones. However, this approach, like the 2D coding technique, becomes unworkable as the frequency of the level change increases. As the length of a given run decreases beyond a certain amount this coding technique may result in expansion rather than compression. Clearly, an eight-bit data word will not efficiently describe a pattern length of less than eight unchanged bits. Thus, even with these known types of data compression excessively large amounts of data are still required.
Not only is equipment that is capable of exceedingly high data rates more expensive, but increasing numbers of data bits to be transferred frequently results in increasing error rates.
Accordingly, it is an object of the present invention to provide an imaging system that avoids or minimizes above-mentioned problems.